mm: Stop kswapd early when nothing's waiting for it to free pages

Keeping kswapd running when all the failed allocations that invoked it
are satisfied incurs a high overhead due to unnecessary page eviction
and writeback, as well as spurious VM pressure events to various
registered shrinkers. When kswapd doesn't need to work to make an
allocation succeed anymore, stop it prematurely to save resources.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
[kdrag0n: Forward-ported from 4.14 to 4.19]
Signed-off-by: Danny Lin <danny@kdrag0n.dev>